Apheresis is a procedure in which individual blood components can be separated and collected from whole blood withdrawn from a subject. Typically, whole blood is withdrawn through a needle inserted into a vein of the subject's arm and into a cell separator, such as a centrifugal bowl. Once the whole blood is separated into its various components, one or more of the components can be removed from the centrifugal bowl. The remaining components can be returned to the subject. In some instances, the remaining components can be returned along with optional compensation fluid to make up for the volume of the removed component. The process of drawing and returning continues until the quantity of the desired component has been collected, at which point the process is stopped. A central feature of apheresis systems is that the processed but unwanted components are returned to the donor. Blood components separated may include, for example, a high density component such as red blood cells, an intermediate density component such as platelets or white blood cells, and a lower density component such as plasma.
As mentioned above, many prior art apheresis systems use a centrifuge bowl to separate and collect the individual blood components. In such systems, the whole blood is drawn into the bowl and separated into the various components (e.g., within a separation region). As the bowl continues to fill with whole blood, red blood cells sediment towards the outer diameter of the bowl, causing a plasma interface (e.g., the interface between the red blood cells and plasma) to move towards the center of the bowl. When the plasma interface reaches a certain point, plasma is pushed out of the bowl and may be collected in one or more collection bags. Blood will continue to fill the bowl until the plasma interface reaches a certain position. At this point, the introduction of whole blood into the bowl is stopped.
After the introduction of whole blood is stopped, the collected plasma may be recirculated to the bowl in order to remove a layer of platelets formed within the bowl. Once the platelets are collected, many prior art systems then collect and/or return the remaining contents of the bowl to the patient. The process is then repeated in a batch-like/intermittent manner until a target amount of blood component (e.g., red blood cells, platelets, plasma, etc.) is collected.